1. Field of the Invention
This invention relates generally to a proppant composite, and more particularly, but not by way of limitation, to an attrition resistant proppant composite for use in hydraulic fracturing.
2. Description of the Related Art
Hydraulic fracturing is commonly used in oil and gas production to access carbon trapped in impermeable geological formations. The process involves injecting a highly pressurized fluid, typically containing water or another carrier, chemicals, and proppants, into a wellbore, which causes the underlying rock to crack. The proppants in the fluid then stay in the cracks in the rock and hold the cracks open, allowing underlying hydrocarbons to flow through the cracks into the wellbore for collection.
Proppants like quartz sand, resin coated sand, ceramics, and materials like bauxite used to make ceramics, for example, are now commonly used in hydraulic fracturing to increase the production of oil and gas from subterranean formations. However, all of these proppants tend to generate dust/fines upon shipping and handling before they are pumped into the well for fracturing. When quartz sand is employed for fracturing, for example, attrition and impingement among quartz particulates and between quartz particulates and the walls of the container occurs during shipping to the fracturing job site. This attrition and impingement are greatly increased during transfer and unloading of the material. Dust/fines are created during bin loading, belt transfer, blender loading, release from multi-sander operations, release from stingers, dust ejection from open fill ports, and other handling operations with potential adverse health and environmental effects. In particular, pneumatic air unloading creates high levels of dust/fines.
Dust/fines are microscopic particulate matter that can be suspended in the air. Such particulate matter occurs naturally, and can also be man-made. This invention is directed to limiting the creation of dust/fines through the use of attrition resistant proppant in transport and hydraulic fracturing and other applications. Respirable particles are a particular concern for the health and safety of workers and other persons who come into contact with dust/fines. These airborne particulates are potentially hazardous because of their ability to penetrate deep into the alveoli of the lungs. In particular, chronic or excessive exposure to respirable crystalline silica such as quartz has been shown to cause pneumoconiosis, commonly known as silicosis.
Within the class of dust/fines, respirable particles are those small enough to enter the alveoli of the lungs and generally include particulates with a diameter of 10 micrometers (or microns) or less. As particle size drops below 10 microns, the probability of particles becoming trapped in the alveoli increases. Although the present invention is successful in the suppression of dust/fines with a diameter of 10 microns or less (including fines with a diameter of 2.5 microns or less), the invention may also suppress other suspended particulate matter that may be larger or sub-micron in size. As greater medical and environmental awareness of the consequence of respirable suspended particles is known, this invention will continue to apply to the suppression of dust/fines in hydraulic fracturing and other applications as those terms may be understood in future practice or regulation.
With regard to the current regulatory environment, the U.S. Occupational Safety and Health Administration (OSHA) is an agency of the U.S. Department of Labor empowered to assure safe and healthful working conditions by setting and enforcing workplace standards. OSHA establishes Permissible Exposure Limits (PELs) for many chemical substances in 29 CFR 1910.1000. OSHA's current PEL for respirable silica dust in General Industry is found in 29 CFR 1910.1000 TABLE Z-3 and the OSHA Technical Manual (OTM) Section II: Chapter 1 Appendix J, Sample Calculations for Crystalline Silica, including the following formulation, Equation 6 from Section III.K.2 of Appendix J:PEL(mg/m3)=(10 mg/m3)/(2+% respirable quarts)Therefore, for a dust containing 100% quartz, the PEL is 10/(100+2), or roughly 0.1 mg/m3. The term “respirable quartz” includes dusts containing greater than one percent quartz with a particle size small enough to reach the alveolar space in the lungs, or less than 10 μm in aerodynamic diameter. Dust exposures are expressed as either a particle concentration (for example, millions of particles per cubic foot of air or mppcf) or a gravimetric concentration (unit mass of particles per volume of air, such as mg/m3). OSHA's regulatory authority is subject to administrative rulemaking process which includes public comment and review. This administrative and political process can result in new or revised standards that take years to be developed, finalized, and promulgated as a standard. The employer's efforts to control silica exposures below the PEL, as addressed by the present invention, will become more difficult if OSHA's proposed rulemaking lowers the PEL to 0.05 milligrams of respirable crystalline silica per cubic meter of air (0.05 mg/m3), as indicated in OSHA's Proposed Rule to the Federal Register on Sep. 12, 2013.
OSHA recognizes that many of its PELs are outdated and that revising the current PELs is a lengthy and complicated process. As such, OSHA recommends that employers consider using alternative occupational exposure limits (i.e., NIOSH Recommended Exposure Limits (RELs) and the ACGIH TLVs). Regarding best industry industrial hygiene practices, the American Conference of Governmental Industrial Hygienists (ACGIH) is a member-based organization dedicated to the industrial hygiene and occupational health and safety industries. The ACGIH annually publishes the ACGIH Guide to Occupational Exposure Values, considered the standard resource for occupational exposure limits in the United States. The ACGIH Threshold Limit Value (TLV) for an eight-hour time weighted average (TWA) workshift exposure to respirable crystalline silica, as included in the 2015 Guide to Occupational Exposure Values and cited in OSHA's 29 CFR 1910.1200 Annotated TABLE Z-3 Mineral Dusts, is 0.025 mg/m3 for α-quartz. The National Institute for Occupational Safety and Health (NIOSH) is part of the Center for Disease Control and Prevention (CDC) within the U.S. Department of Health and Human Services. Among other things, NIOSH is responsible for conducting research and making recommendations for the prevention of work-related injury and illness based on the best available scientific data. The currently published NIOSH Recommended Exposure Level (REL) for a TWA associated with up to a ten-hour workday during a 40-hour workweek is 0.05 mg/m3 for crystalline silica as respirable dust. In addition to these U.S. agencies, foreign agencies are also involved in setting workplace standards and recommendations, including the Scientific Committee on Occupational Exposure Limits (SCOEL) and Institut für Arbeitsschutz der Deutschen Gesetzlichen Unfallversicherung (IFA), which advise the European Commission regarding occupational exposure limits for chemicals in the workplace, and the Workplace Exposure Standards for Airborne Contaminants published in association with the Australian Work Health and Safety Act. The present invention is directed to complying with all these various standards and recommendations, as well as self-imposed standards that may exceed these requirements.
Proppant fines can also cause problems in the recovery of oil and gas. Fines are smaller than whole proppant and thus less effective at propping the cracks open for the oil and gas to flow through. Furthermore, they tend to clog the cracks, inhibiting the flow of hydrocarbons and reducing the productivity of the well.
Previous coated proppants have been aimed primarily at increasing the crush strength of the proppant, with dust control considered only as a secondary benefit. Increasing crush strength can be achieved by coating the proppant with resin. This is very expensive, however, and thus an undesirable solution to increase attrition resistance. Other coatings are aimed at controlling dust by agglomerating small dust particles, rather than preventing dust from forming in the first place.
Based on the foregoing, it is desirable to provide a proppant with low dust/fines subsequent to shipping and handling and, especially, during pneumatic air unloading.
It is further desirable for such a proppant to be a new kind of proppant for the hydraulic fracturing industry.
It is further desirable for such a proppant to allow users of the proppant to be in compliance with OSHA PEL, NIOSH REL, and similar requirements subsequent to shipping and handling and upon pneumatic air unloading, which will better protect workers and prevent nuisance dusting which might disturb the local community near the sand plant, the transload facilities, or the fracturing job site.
It is further desirable for such a proppant to facilitate compliance with a reduced OSHA PEL, NIOSH REL, or similar regulations if required in the future.
It is further desirable for such a proppant to be less expensive to produce than resin coated proppant.
It is further desirable for such a proppant to prevent dust formation rather than solely agglomerating existing dust.